Loudspeaking or handsfree telephone instruments are well-known in the art to enable bidirectional telephone conversations to be undertaken through a microphone and a speaker at the station of one party to a conversation with the station telephone handset "on-hook". For example, such instruments and their operating circuits are shown in U.S. Pat. Nos. 3,725,585 to J. Moniak et al. issued Apr. 3, 1973 and 3,751,602 to R. L. Breeden issued Aug. 7, 1973.
Equally well-known are dual tone multi-frequency (DTMF) generators, as shown in U.S. Pat. Nos. 3,184,554 to Meacham et al. issued May 18, 1965 and 3,447,096 to J. L. Fischer et al. issued May 27, 1969. More recently such generators or push button dialers use digital techniques and are fabricated on an integrated circuit chip such as that sold by American Microsystems, Inc. (AMI) as the S 2859 or that sold by Mostek, Inc. as its MK 5090 series. In U.S. patent application Ser. No. 218,054 filed Dec. 19, 1980 now U.S. Pat. No. 4,406,926 and assigned to the assignee hereof, additions to the AMI circuit were shown. Such integrated circuit DTMF devices receive a selected input from a column and a row of the push button key pad. Using an oscillator, digital counter and ladder networks, the inputs are converted into digitally generated tones transmitted to a tone output terminal. Muting outputs for the receiver and transmitter of the telephone instrument are activated during tone output by logic from the integrated circuit. The chip is powered from the telephone line to which it is connected.
When a loudspeaking or handsfree telephone equipped with a DTMF device is in its normal mode, the device and the speech network are continuously powered from the line over the hookswitch when in its off-hook condition. In the handset (non-handsfree) mode, with the hookswitch on-hook, the telephone instrument is essentially disconnected from the line and there is no power to the instrument internal circuits including speech network.
When the instrument is in the handsfree mode with the hookswitch on-hook--the receiver, transmitter and speech network are all switched off by the hookswitch. The handsfree hybrid is across the line to enable bidirectional speech tranmission. The handsfree or loudspeaking network is powered continuously from an independent power source. The DTMF device is maintained inactive until one of the dialing buttons is depressed to actuate the common mechanical switch (assuming a mechanism as shown in U.S. Pat. No. 3,109,071 issued Oct. 29, 1963 to Mitchell et al.). No such common switch is provided with the integrated circuit digital DTMF devices currently being produced, for example, the S 2859 chip of AMI or variations thereof.